Orthogonal frequency division multiplexing (OFDM) is an efficient modulation scheme for signal transmission over frequency-selective channels. In OFDM, a wide bandwidth is divided into multiple narrowband subcarriers, which are arranged to be orthogonal with each other.
The signals modulated on the subcarriers are transmitted in parallel. For more information, see Cimini, Jr., “Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing,” IEEE Trans. Commun., vol. COM-33, no. 7, July 1985, pp. 665-75; Chuang and Sollenberger, “Beyond 3G: Wideband Wireless Data Access Based on OFDM and Dynamic Packet Assignment,” IEEE Communications Magazine, Vol. 38, No. 7, pp. 78-87, July 2000.
One way to use OFDM to support multiple access for multiple subscribers is through time division multiple access (TDMA), in which each subscriber uses all the subcarriers within its assigned time slots. Orthogonal frequency division multiple access (OFDMA) is another method for multiple access, using the basic format of OFDM. In OFDMA, multiple subscribers simultaneously use different subcarriers, in a fashion similar to frequency division multiple access (FDMA). For more information, see Sari and Karam, “Orthogonal Frequency-Division Multiple Access and its Application to CATV Networks,” European Transactions on Telecommunications, Vol. 9 (6), pp. 507-516, November/December 1998 and Nogueroles, Bossert, Dander, and Zyablov, “Improved Performance of a Random OFDMA Mobile Communication System,” Proceedings of IEEE VTC '98, pp. 2502-2506.
Multipath causes frequency-selective fading. The channel gains are different for different subcarriers. Furthermore, the channels are typically uncorrelated for different subscribers. The subscribers that are in deep fade for one subscriber may provide high channel gains for another subscriber. Therefore, it is advantageous in an OFDMA system to adaptively allocate the subcarriers to subscribers so that each subscriber enjoys a high channel gain. For more information, see Wong et al., “Multiuser OFDM with Adaptive Subcarrier, Bit and Power Allocation,” IEEE J. Select. Areas Commun., Vol. 17(10), pp. 1747-1758, October 1999.
Within one cell, the subscribers can be coordinated to have different subcarriers in OFDMA. The signals for different subscribers can be made orthogonal and there is little intracell interference. However, with aggressive frequency reuse plan, e.g., the same spectrum is used for multiple neighboring cells, the problem of intercell interference arises. It is clear that the intercell interference in an OFDMA system is also frequency selective and it is advantageous to adaptively allocate the subcarriers so as to mitigate the effect of intercell interference.
One approach to subcarrier allocation for OFDMA is a joint optimization operation, not only requiring the activity and channel knowledge of all the subscribers in all the cells, but also requiring frequent rescheduling every time an existing subscriber is dropped off the network or a new subscriber is added onto the network. This is often impractical in real wireless system, mainly due to the bandwidth cost for updating the subscriber information and the computation cost for the joint optimization.